B B C C E E BIJUNCTION TRANSISTOR. A transistor is a semiconductor device used to amplify and...
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Transcript of B B C C E E BIJUNCTION TRANSISTOR. A transistor is a semiconductor device used to amplify and...
B
C
E
BIJUNCTION TRANSISTOR
A transistor is a semiconductor device used to amplify and switch electronic signals.
A transistor is made up of three layers – an ‘n’ layer sandwiched between two ‘p’ layers or a ‘p’ layer between two ‘n’ layers.
Doping of each layer is different and that is what is responsible for the operation(amplification).
Transistor
The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors:
N P N
n p nE
B
C
Cross Section
B
C
E
Schematic Symbol
P N P
p n pE
B
C
Cross Section
B
C
E
Schematic Symbol
Collector is moderately doped
Base is lightly doped Emitter is heavily doped
Click to view NPN Transistor
Click to view PNP Transistor
BJT Relationships - Equations
B
CE
IE IC
IB
-
+
VBE VBC
+
-
+- VCE
B
CEIE IC
IB-
+VEB VCB
+
-
+ -VEC
npnIE = IB + IC
VCE = -VBC + VBE
pnpIE = IB + IC
VEC = VEB - VCB
DC and DC
= Common-emitter current gain = Common-base current gain = IC = IC IB IE
The relationships between the two parameters are: = = + 1 1 -
Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.
Modes of OperationMost important mode of operationCentral to amplifier operationEmitter –Base junction Forward biased and
Collector –base Reverse Biased
Active:
Saturation: Barrier potential of the junctions cancel each other out causing a virtual short
Ideal transistor behaves like an closed switch
Both junction are Forward biased
Cutoff: Current reduced to zeroIdeal transistor behaves like an open
switchBoth junction are Reverse biased
Three Types of BJT Configurations
Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.
Common-Base Biasing (CB) input = VEB & IE
output = VCB & ICCommon-Emitter Biasing (CE)
input = VBE & IB
output = VCE & ICCommon-Collector Biasing (CC)
input = VBC & IBoutput = VEC & IE
BJT Transconductance Curve
Typical NPN Transistor
VBE
IC
2 mA
4 mA
6 mA
8 mA
0.7 V
Collector Current:
Transconductance: (slope of the curve)gm = IC / VBE
IES = The reverse saturation current of the B-E Junction.VT = kT/q = 26 mV (@ T=300K) = the emission coefficient and is usually 1
BEC T ES
T
VI I e
V
Common-BaseCircuit Diagram: NPN Transistor
Region of Operation IC VCE VBE VCB
C-B Bias
E-B Bias
Active βIB =VBE+VCE 0.7V 0V Rev. Fwd.
Saturation Max 0V 0.7V -0.7V<VCE<0 Fwd. Fwd.
Cutoff 0 =VBE+VCE 0V 0V Rev. None/Rev.
The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.
Common-Base input characteristics
Input characteristics for the CB configuration gives relation between the input quantities, input voltage VEB and input current IE for fixed VCB values
The input circuit in CB configuration involves the emitter-base diode, which is forward biased in active region. Therefore, the relationship between VEB and IE is nothing but the forward characteristics of a diode
Common-Base input characteristicsIn the above characteristics, VCB = Open represents the characteristics of the forward biased emitter
With increase of VCB, the curves shift downwards i.e., we get the same IE with less VEB. This is because, from the early effect increases the IE increases with VEB held constant
Common-Base out put characteristics Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works.
Emitter-Current Curves
Common-EmitterCircuit Diagram
+_VCC
IC
VCE
IB
Collector-Current Curves
VCE
IC
Active Region
IB
Saturation RegionCutoff RegionIB = 0
Region of Operation
Description
Active Small base current controls a large collector current
Saturation VCE(sat) 0.2V, VCE increases with IC
Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.
The input quantities for C.E. configuration are base current IB and base emitter voltage VBE
The input characteristics curves are in between IB and VBE for various values of collector to emitter voltage VCE
If VCE = 0 and if the base-emitter junction is forward biased, the input characteristics is the same as the characteristics of forward biased diodeIf VCE is increased then VCB increases By applying KVL around the transistor
If VCE is increased then VCB increases
BE CE CB
CE CB BE
V - V +V =0
V =V +V
Common Emitter Input Characteristics
Common Emitter Input Characteristics
Increase in VCB leads to decrease in effective base width WB
| due to early effect, resulting in decrease of recombination and consequently, decrease in base current due to recombination.
CE CB B
CE
V V I
The curves move right side as V increases
Common Emitter output Characteristics
The output quantities in C.E. configuration are IC and VCE the o/p characteristics gives a relationship between IC and VCE with base current IB as a parameter.This family of curves may be divided into three regions those are active region, saturation region and cutoff region.
Common-CollectorIt is often called an emitter follower since its output is taken from the emitter resistor.Is useful as an impedance matching device since its input impedance is much higher than its output impedance.It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.
Common-Collector
Emitter-Current Curves
VCE
IE
Active
Region
IB
Saturation Region Cutoff Region
IB = 0
The Common-collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC
as a function of VCE and
IB we are looking at IE.Also, since 1, and = IC/IE that means IC
IE
Common collector input Characteristics
Input Characteristics As VCB increases according to early effect base width decreases and IB decreases.
Common collector output Characteristics
Output Characteristics: The common-collector circuit is basically same as the common-emitter, with the exception that the load resistor is in the emitter circuit, the output characteristics are similar to that of CE configuration. It is because E C I I
Transistor as amplifier
Transistor amplifies current as well as voltage and is a current operated device.
The CE configuration is widely used as it amplifies current and voltage unlike the other configurations.
Click to view Image
PARAMETER CB CE CC
Input impedance (Ri)
Output Impedance(Ro)
D.C current gain
Voltage gain Very Large Moderate
Applications For High Frequency
For Audio frequency For impedance matching
Phase relationship between i/p and o/p
In- phase Out-of phase In- phase
EB
E
V (Low) 10
I
( )BE
B
V Moderate 1K
I
( )CB
B
VHigh 100K
I
CB
C
V (High) 100k
I
( )CB
C
V Moderate 10K
I
( ) 10EC
E
V Low
I
C
E
I 1I
C
B
I = High value (50-500)
I
E
B
I = (51-501)
I
very low 1
COMPARISION of CB, CE AND CC PARAMETERS
Transistor summary